Land Applications and Surface Disposal

Land application of sludge is defined as the beneficial use of the sludge at agronomic rates; all other land displacements are considered as surface

Compressed air

Compressed air

Figure 7.12. A schematic diagram of a rotary dryer for sludge dewatering.
Figure 7.13. A schematic diagram of a multieffect evaporator for sludge de-watering.

disposal of sludge. The detailed regulation regarding land application and surface disposal of sludge from wastewater treatment plants are discussed in 1993 EPA 40 CFR Part 503. It stipulates that landfilling of sludge is considered as "beneficial use" only when such disposal includes methane gas recovery for fuel. However, methane operations are relatively rare. Alternative beneficial uses are receiving greater attention because of a decline in available landfill space and an interest in conserving nutrients, and utilizing soil conditioning properties and other recoverable qualities of sludge. Thus, land application for soil conditioning and fertilization is the primary beneficial use of sludge from food and agricultural wastewater treatment plants.

Sludge may be disposed of by liquid injection to land or by disposal in a landfill. There are concerns about sludge incineration because of air pollutants in the emissions, along with the high cost of supplemental fuel, making this a less attractive and less commonly constructed means of sludge treatment and disposal. There is no process that completely eliminates the requirements for disposal of sludges (there is ash from incineration that needs to be properly disposed of).

Similar to land application of wastewater, land application of sludge is also affected greatly by characteristics of wastes, including organic contents (volatile solids), nutrients, pathogens, metals, and toxic organics, and soil characteristics. Metcalf and Eddy, Inc. (1991) and Reed et al. (1995) provide the concise procedures regarding site selection, process design, and land application and surface disposal methods. The reader is encouraged to explore all aspects of sludge management issues in these references.

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